1. Field of the Invention
This invention relates to the field of rail-to-rail amplifiers, and particularly to methods of establishing the transition threshold between the differential input pairs of a rail-to-rail amplifier.
2. Description of the Related Art
A rail-to-rail amplifier""s input stage typically includes complementary differential pairs. One differential pair conducts when the common mode voltage Vcm applied to the amplifier""s inputs is above a particular threshold voltage Vth, and the other pair conducts when Vcm is less than Vth.
A typical bipolar rail-to-rail input stage is shown in FIG. 1. A PNP differential pair Q1 and Q2 have their emitters connected to a common mode node 10, and an NPN differential pair Q3 and Q4 have their emitters connected to a common mode node 12. The bases of Q1 and Q3 are connected to receive one side (VN) of a differential input signal, and the bases of Q2 and Q4 are connected to receive the other side (VP) of the input signal. Input stage transistors Q1, Q2, Q3 and Q4 produce currents IPP, IPN, INP and INN, respectively, in response to a differential voltage applied at VN and VP. To form a complete rail-to-rail amplifier, currents IPP, IPN, INP and INN are routed to an output stage (not shown) which produces the amplifier""s output voltage.
This type of input stage typically includes a tail current source 14 which provides a tail current Itail to the input stage transistors. The input stage is arranged such that Itail is provided to common mode node 10 when the common mode input voltage       V    cm    ⁢      (          =                        VN          +          VP                2              )  
is less than Vth, and to common mode node 12 when Vcm greater than Vth.
One possible mechanism for steering Itail to the proper node uses a steering transistor QS, which is connected between node 10 and a pair of transistors Qm1 and Qm2 which form a current mirror that mirrors the current in QS to common mode node 12. Threshold voltage Vth is applied to the base of QS. When arranged in this way, when Vcm less than  less than Vth, QS is off, Itail is provided to common mode node 10, and Q1 and Q2 are active. When Vcm greater than  greater than Vth, QS is on, Itail is provided to common mode node 12, and Q3 and Q4 are active.
Unfortunately, as the common-mode input voltage crosses Vth, a discontinuity appears in the output due to crossover distortion. The threshold voltage is generally skewed toward one rail or the other in order to yield the widest distortion-free input range. However, unless the amplifier receives a input voltage which operates over a known, limited range, crossover distortion will inevitably occur.
A bipolar rail-to-rail input stage is presented which overcomes the problem noted above. A user is allowed to select a desired transition threshold voltage via a xe2x80x9cselectxe2x80x9d signal, thereby providing a selectable crossover-distortion-free common mode input voltage range.
The input stage includes complementary differential input pairs, and a switching circuit which makes one or the other of the input pairs active depending on the relationship between a transition threshold voltage Vth and the common mode input voltage Vcm. The present input stage also includes a transition threshold voltage selection circuit which provides a selectable one of at least two different Vth voltages to the switching circuit in response to a select signal.
The present input stage is preferably arranged such that it can be turned off in response to a disable signal. In this embodiment, the select signal has first, second, and third states which correspond to logic xe2x80x9chighxe2x80x9d, logic xe2x80x9clowxe2x80x9d, and xe2x80x9cfloatingxe2x80x9d states. The transition threshold voltage selection circuit is arranged to provide a first Vth voltage to the switching circuit when the select signal is in a first state, to provide a second Vth voltage when the select signal is in a second state, and to assert the disable signal and thereby turn the input stage off when the select signal is in a third state.
Further features and advantages of the invention will be apparent to those skilled in the art from the following detailed description, taken together with the accompanying drawings.